Aluminium is produced conventionally by the Hall-Heroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures up to around 950.degree. C. A Hall-Heroult reduction cell typically has a steel shell provided with an insulating lining of refractory material, which in turn has a lining of carbon which contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode forming the cell bottom floor. The cathode is usually an anthracite or graphite based carbon lining made of prebaked cathode blocks, joined with a ramming mixture or glue.
In Hall-Heroult cells, a molten aluminium pool acts as the cathode surface. The carbon bottom lining or cathode material has a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminium as well as penetration of sodium into the carbon, which by chemical reaction and intercalation causes swelling, deformation and disintegration of the cathode carbon blocks and ramming mix. In addition, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides.
Difficulties in operation also arise from the accumulation of undissolved alumina sludge on the surface of the carbon cathode beneath the aluminium pool which forms insulating regions on the cell bottom. Penetration of cryolite and aluminium through the carbon body and the deformation of the cathode carbon blocks also cause displacement of such cathode blocks. Due to displacement of the cathode blocks and the formation of cracks, aluminium reaches the steel cathode conductor bars causing corrosion thereof leading to deterioration of the electrical contact, non uniformity in current distribution and an excessive iron content in the aluminium metal produced.
Extensive research has been carried out with Refractory Hard Metals (RHM) such as TiB.sub.2 as cathode materials. TiB.sub.2 and other RHM's are practically insoluble in aluminium, have a low electrical resistance, and are wetted by aluminium. This should allow aluminium to be electrolytically deposited directly on an RHM cathode surface, and should avoid the necessity for a deep aluminium pool.
Because titanium diboride and similar Refractory Hard Metals are wettable by aluminium, resistant to the corrosive environment of an aluminium production cell, and are good electrical conductors, numerous cell designs utilizing Refractory Hard Metal have been proposed, which would present many advantages, notably including the saving of energy by reducing the anode-cathode distance (ACD).
U.S. Pat. No. 3,856,650 proposed lining a carbon cell bottom with a ceramic coating upon which parallel rows of tiles are placed, in the molten aluminium, and spaced apart from one another by expansion gaps in a grating-like arrangement. The purpose of this "grating" was to protect the ceramic coating against mechanical effects due, for example, to movements of the aluminium pool.
U.S. Pat. No. 4,243,502 described designs for aluminium-wettable cathodes some of which had a generally horizontal active surface supported by one or more supporting plates, usually connected to a current supply by an extension protruding from the top of the electrolyte, between the anodes. Such designs were not practicable.
U.S. Pat. No. 4,410,412 described wettable cathodes made of aluminide materials. These cathodes were supposed to be exchangeable, by holding several cathode elements together in a holder of insoluble refractory material. Special cell designs to make use of such aluminides were also described in U.S. Pat. No. 4,462,886. Again, such materials and designs did not prove to be practicable.
PCT patent application W083/04271 proposed cathodic elements of refractory hard materials such as titanium diboride in the shape of mushrooms having relatively large flat tops facing the anode in order to maximize the active cathode surface. However, no adequate means could be found for connecting the mushroom stems to the cell bottoms, so this design also failed.
To accommodate for fluctuations in the level of the pool of aluminium, European patent EP-B-0 082 096 proposed the use of floating cathode elements made of titanium diboride combined with a lighter material to reduce its density, for instance graphite. These floating elements were restrained by elements connected to the cell bottom, leading to an impractical design.
EP-A-0 103 350 proposed the use of tubular cathode elements, for example of titanium diboride, which rest on the cell bottom dipping in a shallow aluminium pool. The inner diameter of the elements was such as to maintain molten aluminium up to near the tops of the tubes by capillary action. These individual tubes were distributed over the cell bottom with a suitable spacing, and were to remain on the cell bottom during use.
U.S. Pat. No. 4,349,427 has proposed replaceable modular cathode assemblies in the form of a table on which free shapes of refractory material are packed.
To restrict movement in a "deep" cathodic pool of molten aluminium, U.S. Pat. No. 4,824,531 proposed filling the cell bottom with a packed bed of loose pieces of refractory material. Such a design has many potential advantages but, because of the risk of forming a sludge by detachment of particles from the packed bed, the design has not found acceptance.
U.S. Pat. No. 4,443,313 sought to avoid the disadvantages of the previously mentioned loose packed bed by providing a monolayer of closely packed small ceramic shapes such as balls, tubes or honeycomb tiles having uniform, small apertures that restrain the entry of sludge.
Despite extensive efforts and the potential advantages of having surfaces of titanium diboride at the cell cathode bottom, such propositions have not been commercially adopted by the aluminium industry.
Recently, a number of proposals have been made for the feasible, low-cost production of various composite materials containing or coated with titanium diboride or other refractory ceramic materials, enabling promising applications in many of the already-proposed cell designs.
For instance, WO 93/20027 discloses forming protective refractory coatings on a conductive substrate like carbon starting from a micropyretic reaction layer from a slurry containing reactants in a colloidal carrier. WO 93/20026 discloses protective coatings applied from a colloidal slurry containing particulate reactant or non-reactant substances. WO 93/25731 more particularly describes the application of pre-formed refractory borides in a colloidal carrier to carbon cell components of aluminium production cells.
Such coating materials have in particular enabled substantial improvements in the conventional cell bottom designs. However, it has turned out that many of the heretofore proposed "new" cell designs are unsatisfactory in one or more respects, even with materials that stand up in the environment.